1
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Chau AL, Karnaukh KM, Maskiewicz I, Read de Alaniz J, Pitenis AA. Photoresponsive hydrogel friction. SOFT MATTER 2024; 20:7227-7236. [PMID: 39225393 DOI: 10.1039/d4sm00677a] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 09/04/2024]
Abstract
Photoresponsive hydrogels are an emerging class of stimuli-responsive materials that exhibit changes in physical or chemical properties in response to light. Previous investigations have leveraged photothermal mechanisms to achieve reversible changes in hydrogel friction, although few have focused on photochemical means. To date, the tribological properties of photoswitchable hydrogels (e.g., friction and lubrication) have remained underexplored. In this work, we incorporated photoresponsive methoxy-spiropyran-methacrylate monomers (methoxy-SP-MA) into a hydrogel network to form a copolymerized system of poly(N-isopropylacrylamide-co-2-acrylamido-2-methylpropane sulfonic acid-co-methoxy-spiropyran-methacrylate) (p(NIPAAm-co-AMPS-co-SP)). We demonstrated repeatable photoresponsive changes to swelling, friction, and stiffness over three light cycles. Our findings suggest that volume changes driven by the decreased hydrophilicity of the methoxy-SP-MA upon light irradiation are responsible for differences in the mechanical and tribological properties of our photoresponsive hydrogels. Our results could inform future designs of photoswitchable hydrogels for applications ranging from biomedical applications to soft robotics.
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Affiliation(s)
- Allison L Chau
- Materials Department, University of California, Santa Barbara, Santa Barbara, CA, USA.
| | - Kseniia M Karnaukh
- Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, CA, USA.
| | - Ian Maskiewicz
- Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, CA, USA.
| | - Javier Read de Alaniz
- Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, CA, USA.
| | - Angela A Pitenis
- Materials Department, University of California, Santa Barbara, Santa Barbara, CA, USA.
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2
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Zhang Z, Shen C, Zhang P, Xu S, Kong L, Liang X, Li C, Qiu X, Huang J, Cui X. Fundamental, mechanism and development of hydration lubrication: From bio-inspiration to artificial manufacturing. Adv Colloid Interface Sci 2024; 327:103145. [PMID: 38615561 DOI: 10.1016/j.cis.2024.103145] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/11/2023] [Revised: 03/26/2024] [Accepted: 03/30/2024] [Indexed: 04/16/2024]
Abstract
Friction and lubrication are ubiquitous in all kinds of movements and play a vital role in the smooth operation of production machinery. Water is indispensable both in the lubrication systems of natural organisms and in hydration lubrication systems. There exists a high degree of similarity between these systems, which has driven the development of hydration lubrication from biomimetic to artificial manufacturing. In particular, significant advancements have been made in the understanding of the mechanisms of hydration lubrication over the past 30 years. This enhanced understanding has further stimulated the exploration of biomimetic inspiration from natural hydration lubrication systems, to develop novel artificial hydration lubrication systems that are cost-effective, easily transportable, and possess excellent capability. This review summarizes the recent experimental and theoretical advances in the understanding of hydration-lubrication processes. The entire paper is divided into three parts. Firstly, surface interactions relevant to hydration lubrication are discussed, encompassing topics such as hydrogen bonding, hydration layer, electric double layer force, hydration force, and Stribeck curve. The second part begins with an introduction to articular cartilage in biomaterial lubrication, discussing its compositional structure and lubrication mechanisms. Subsequently, three major categories of bio-inspired artificial manufacturing lubricating material systems are presented, including hydrogels, polymer brushes (e.g., neutral, positive, negative and zwitterionic brushes), hydration lubricant additives (e.g., nano-particles, polymers, ionic liquids), and their related lubrication mechanism is also described. Finally, the challenges and perspectives for hydration lubrication research and materials development are presented.
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Affiliation(s)
- Zekai Zhang
- Center for Advanced Jet Engineering Technologies (CaJET), Key Laboratory of High Efficiency and Clean Mechanical Manufacture of Ministry of Education, School of Mechanical Engineering, Shandong University, Jinan, Shandong 25006, China
| | - Chaojie Shen
- Center for Advanced Jet Engineering Technologies (CaJET), Key Laboratory of High Efficiency and Clean Mechanical Manufacture of Ministry of Education, School of Mechanical Engineering, Shandong University, Jinan, Shandong 25006, China
| | - Peipei Zhang
- Advanced Interdisciplinary Technology Research Center, National Innovation Institute of Defense Technology, Beijing 100071, China
| | - Shulei Xu
- Center for Advanced Jet Engineering Technologies (CaJET), Key Laboratory of High Efficiency and Clean Mechanical Manufacture of Ministry of Education, School of Mechanical Engineering, Shandong University, Jinan, Shandong 25006, China
| | - Lingchao Kong
- Advanced Interdisciplinary Technology Research Center, National Innovation Institute of Defense Technology, Beijing 100071, China
| | - Xiubing Liang
- Advanced Interdisciplinary Technology Research Center, National Innovation Institute of Defense Technology, Beijing 100071, China
| | - Chengcheng Li
- Advanced Interdisciplinary Technology Research Center, National Innovation Institute of Defense Technology, Beijing 100071, China
| | - Xiaoyong Qiu
- Center for Advanced Jet Engineering Technologies (CaJET), Key Laboratory of High Efficiency and Clean Mechanical Manufacture of Ministry of Education, School of Mechanical Engineering, Shandong University, Jinan, Shandong 25006, China
| | - Jun Huang
- Center for Advanced Jet Engineering Technologies (CaJET), Key Laboratory of High Efficiency and Clean Mechanical Manufacture of Ministry of Education, School of Mechanical Engineering, Shandong University, Jinan, Shandong 25006, China.
| | - Xin Cui
- Advanced Interdisciplinary Technology Research Center, National Innovation Institute of Defense Technology, Beijing 100071, China.
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3
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Wang H, Wang Q, Su Y, Wang J, Zhang X, Liu Y, Zhang J. Thermosensitive Triblock Copolymer for Slow-Release Lubricants under Ocular Conditions. ACS APPLIED MATERIALS & INTERFACES 2024; 16:1675-1687. [PMID: 38127457 DOI: 10.1021/acsami.3c12389] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/23/2023]
Abstract
The ocular environment is crucial for a biological lubrication system. An unstable condition of tear film may cause a series of ocular diseases due to serious friction, such as dry eye syndrome, which has drawn extensive attention nowadays. In this study, an in vitro biocompatible superlubricity system, containing thermogelling copolymers (PCGA-PEG-PCGA) and slow-release lubricant (PEG 300/Tween 80), was constructed. First, the sol-gel transition temperature and gel strength of PCGA-PEG-PCGA were adjusted based on the ocular environment by regulating the length of PCGA blocks. Furthermore, the copolymer hydrogel exhibited a reliable slow-release property within 10 days and showed low cytotoxicity. Then, the superlubricity (coefficient of friction of approximately 0.005) was achieved with its released PEG 300/Tween 80 aqueous solution at the sliding velocity range of 1-100 mm s-1 and pressure range of 10-22 kPa. However, the lubrication behaviors varied, while PEG 300 chains and Tween 80 micelles were demonstrated to form a multilayer and a single layer adsorption structure on the sliding surface, respectively. On the whole, the composite lubrication systems, especially the one composed of Tween 80, showed excellent tribological properties owing to the stable slow-release and full hydration effects under ocular conditions, which hold great potential for improving ocular lubrication and maintaining human visual health.
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Affiliation(s)
- Hongdong Wang
- School of Mechatronic Engineering and Automation, Shanghai University, Shanghai 200444, China
- Key Laboratory of Advanced Display and System Application, Ministry of Education, Shanghai 200444, China
- State Key Laboratory of Tribology in Advanced Equipment, Tsinghua University, Beijing 100084, China
| | - Qi Wang
- School of Mechatronic Engineering and Automation, Shanghai University, Shanghai 200444, China
- Key Laboratory of Advanced Display and System Application, Ministry of Education, Shanghai 200444, China
| | - Yunjuan Su
- School of Mechatronic Engineering and Automation, Shanghai University, Shanghai 200444, China
- Key Laboratory of Advanced Display and System Application, Ministry of Education, Shanghai 200444, China
| | - Junyu Wang
- State Key Laboratory of Tribology in Advanced Equipment, Tsinghua University, Beijing 100084, China
| | - Xiacong Zhang
- Department of Polymer Materials, School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China
| | - Yuhong Liu
- State Key Laboratory of Tribology in Advanced Equipment, Tsinghua University, Beijing 100084, China
| | - Jianhua Zhang
- Key Laboratory of Advanced Display and System Application, Ministry of Education, Shanghai 200444, China
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4
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Mees J, O'Connor TC, Pastewka L. Entropic stress of grafted polymer chains in shear flow. J Chem Phys 2023; 159:094902. [PMID: 37668251 DOI: 10.1063/5.0158245] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/16/2023] [Accepted: 08/14/2023] [Indexed: 09/06/2023] Open
Abstract
We analyze the shear response of grafted polymer chains in shear flow via coarse-grained molecular dynamics simulations with an explicit solvent. We find that the solvent flow penetrates into almost the whole brush for "mushroom"-type brushes but only a few bond distances for dense brushes. In all cases, the external stress on the wall equals the entropic stress associated with the distorted polymer conformations. We find that the external stress increases linearly with shear rate at low rates and sublinearly at high rates. The transition from linear to sublinear scaling occurs where chains react to flow by reorienting. Sublinear scaling with shear rate disappears if the shear rate is nondimensionalized with the effective relaxation time of chain subsegments located in the outer part of the brush that experiences flow.
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Affiliation(s)
- Jan Mees
- Department of Microsystems Engineering, University of Freiburg, Georges-Köhler-Allee 103, Freiburg 79110, Germany
- Cluster of Excellence LivMatS, Freiburg Center for Interactive Materials and Bioinspired Technologies, University of Freiburg, Georges-Köhler-Allee 105, Freiburg 79110, Germany
| | - Thomas C O'Connor
- Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, USA
| | - Lars Pastewka
- Department of Microsystems Engineering, University of Freiburg, Georges-Köhler-Allee 103, Freiburg 79110, Germany
- Cluster of Excellence LivMatS, Freiburg Center for Interactive Materials and Bioinspired Technologies, University of Freiburg, Georges-Köhler-Allee 105, Freiburg 79110, Germany
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5
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Lee MJ, Espinosa-Marzal RM. Intrinsic and Extrinsic Tunability of Double-Network Hydrogel Strength and Lubricity. ACS APPLIED MATERIALS & INTERFACES 2023; 15:20495-20507. [PMID: 37053001 PMCID: PMC10141240 DOI: 10.1021/acsami.3c00949] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 01/20/2023] [Accepted: 03/31/2023] [Indexed: 06/19/2023]
Abstract
Double-network (DN) hydrogels are promising materials for tissue engineering due to their biocompatibility, high strength, and toughness, but understanding of their microstructure-property relationships still remains limited. This work investigates a DN hydrogel comprising a physically crosslinked agarose, as the first network, and a chemically crosslinked copolymer with a varying ratio of acrylamide and acrylic acid, as the second network. The charge, intrinsic to most DN hydrogels, introduces a responsive behavior to chemical and electrical stimuli. The DN strengthens agarose hydrogels, but the strengthening decreases with the swelling ratio resulting from increasing acrylic acid content or reducing salt concentration. Through careful imaging by atomic force microscopy, the heterogenous surface structure and properties arising from the DN are resolved, while the lubrication mechanisms are elucidated by studying the heterogeneous frictional response to extrinsic stimuli. This method reveals the action of the first (agarose) network (forming grain boundaries), copolymer-rich and poor regions (in grains), charge and swelling in providing lubrication. Friction arises from the shear of the polymeric network, whereas hydrodynamic lift and viscoelastic deformation become more significant at higher sliding velocities. We identify the copolymer-rich phase as the main source of the stimulus-responsive behavior. Salt concentration enhances effective charge density and reduces viscoelastic deformation, while electric bias swells the gel and improves lubrication. This work also demonstrates the dynamic control of interfacial properties like hydrogel friction and adhesion, which has implications for other areas of study like soft robotics and tissue replacements.
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Affiliation(s)
- Ming Jun Lee
- Department
of Materials Science and Engineering, University
of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States
| | - Rosa M. Espinosa-Marzal
- Department
of Materials Science and Engineering, University
of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States
- Department
of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, United States
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6
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Composition controls soft hydrogel surface layer dimensions and contact mechanics. Biointerphases 2022; 17:061002. [DOI: 10.1116/6.0002047] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022] Open
Abstract
Hydrogels are soft hydrated polymer networks that are widely used in research and industry due to their favorable properties and similarity to biological tissues. However, it has long been difficult to create a hydrogel emulating the heterogeneous structure of special tissues, such as cartilage. One potential avenue to develop a structural variation in a hydrogel is the “mold effect,” which has only recently been discovered to be caused by absorbed oxygen within the mold surface interfering with the polymerization. This induces a dilute gradient-density surface layer with altered properties. However, the precise structure of the gradient-surface layer and its contact response have not yet been characterized. Such knowledge would prove useful for designs of composite hydrogels with altered surface characteristics. To fully characterize the hydrogel gradient-surface layer, we created five hydrogel compositions of varying monomer and cross-linker content to encompass variations in the layer. Then, we used particle exclusion microscopy during indentation and creep experiments to probe the contact response of the gradient layer of each composition. These experiments showed that the dilute structure of the gradient layer follows evolving contact behavior allowing poroelastic squeeze-out at miniscule pressures. Stiffer compositions had thinner gradient layers. This knowledge can potentially be used to create hydrogels with a stiff load-bearing bulk with altered surface characteristics tailored for specific tribological applications.
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7
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Mostakhdemin M, Nand A, Ramezani M. Tribological Evaluation of Silica Nanoparticle Enhanced Bilayer Hydrogels as A Candidate for Cartilage Replacement. Polymers (Basel) 2022; 14:polym14173593. [PMID: 36080668 PMCID: PMC9460628 DOI: 10.3390/polym14173593] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/24/2022] [Revised: 08/20/2022] [Accepted: 08/23/2022] [Indexed: 11/24/2022] Open
Abstract
Polymeric hydrogels can be used as artificial replacement for lesioned cartilage. However, modulating the hydrogel formulation that mimics articular cartilage tissue with respect to mechanical and tribological properties has remained a challenge. This study encompasses the tribological evaluation of a silica nanoparticle (SNP) loaded bilayer nanocomposite hydrogel (NCH), synthesized using acrylamide, acrylic acid, and alginate via modulated free-radical polymerization. Multi-factor pin-on-plate sliding wear experiments were carried out with a steel ball counterface using a linear reciprocating tribometer. Tribological properties of NCHs with 0.6 wt% SNPs showed a significant improvement in the wear resistance of the lubricious layer and a low coefficient of friction (CoF). CoF of both non-reinforced hydrogel (NRH) and NCH at maximum contact pressure ranged from 0.006 to 0.008, which is in the order of the CoF of healthy articular cartilage. Interfacial surface energy was analysed according to Johnson, Kendall, and Robert’s theory, and NCHs showed superior mechanical properties and surface energy compared to NRHs. Lubrication regimes’ models were drawn based on the Stribeck chart parameters, and CoF results were highlighted in the elastoviscous transition regime.
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Affiliation(s)
- Mohammad Mostakhdemin
- Department of Mechanical Engineering, Auckland University of Technology, Auckland 1010, New Zealand
- Correspondence: (M.M.); (M.R.)
| | - Ashveen Nand
- Faculty of Engineering, University of Auckland, Auckland 1010, New Zealand
| | - Maziar Ramezani
- Department of Mechanical Engineering, Auckland University of Technology, Auckland 1010, New Zealand
- Correspondence: (M.M.); (M.R.)
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8
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Chen L, Hu W, Du M, Song Y, Wu Z, Zheng Q. Bioinspired, Recyclable, Stretchable Hydrogel with Boundary Ultralubrication. ACS APPLIED MATERIALS & INTERFACES 2021; 13:42240-42249. [PMID: 34436862 DOI: 10.1021/acsami.1c12631] [Citation(s) in RCA: 18] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/13/2023]
Abstract
Although hydrogels exhibit excellent low frictional behavior, their friction coefficients cannot meet the requirements for biology, especially at low sliding velocities. Inspired by the natural lubrication mechanism from animals, plants, or even microorganisms, a nonionic surfactant, Tween 80, was introduced into a biofriendly poly(vinyl alcohol) (PVA) hydrogel to construct a composite hydrogel with ultralubrication. Such a combination endows PVA hydrogels with an ultralow coefficient of friction (10-3 to 10-4) under an extremely low sliding velocity (0.01 mm/s). Tween 80 micelles and aggregates, together with hydrophobic molds, induce rough surfaces and high carbon contents on the surface of the hydrogel, promoting excellent lubrication behavior of the composite hydrogel. In addition to the desirable lubrication, this environmentally friendly composite hydrogel also exhibited excellent flexibility at subzero temperatures, tensile properties, and good recyclability. Additionally, the method of introducing Tween 80 into hydrogels to reduce friction is also effective in chemically crosslinked double-network hydrogels.
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Affiliation(s)
- Lu Chen
- MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
| | - WenXuan Hu
- MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
| | - Miao Du
- MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
| | - Yihu Song
- MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
| | - Ziliang Wu
- MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
| | - Qiang Zheng
- MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China
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9
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Kodakkadan YNV, Maslen C, Cigler P, Štěpánek F, Rehor I. Friction-directed self-assembly of Janus lithographic microgels into anisotropic 2D structures. J Mater Chem B 2021; 9:4718-4725. [PMID: 34076033 DOI: 10.1039/d1tb00572c] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/11/2022]
Abstract
We present a method for creating ordered 2D structures with material anisotropy from self-assembling micro-sized hydrogel particles (microgels). Microgel platelets of polygonal shapes (hexagon, square, and rhombus), obtained by a continuous scalable lithographic process, are suspended in an aqueous environment and sediment on an inclined plane. As a consequence of gravitational pull, they slide over the plane. Each half of the microgel is composed of a different type of hydrogel [poly(N-isopropylacrylamide) (PNIPAM), and poly(ethylene glycol) diacrylate (PEGDA), respectively] which exhibit different frictional coefficients when sheared over a substrate. Hence the microgels self-orientate as they slide, and the side with the lower frictional coefficient positions in the direction of sliding. The self-oriented microgels concentrate at the bottom of the tilted plane. Here they form densely packed structures with translational as well as orientational order.
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Affiliation(s)
| | - Charlie Maslen
- University of Chemistry and Technology Prague, Faculty of Chemical Engineering, Technicka 5, 166 28 Prague 6, Czech Republic.
| | - Petr Cigler
- Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences, Flemingovo nam. 2, 160 00, Prague, Czech Republic
| | - František Štěpánek
- University of Chemistry and Technology Prague, Faculty of Chemical Engineering, Technicka 5, 166 28 Prague 6, Czech Republic.
| | - Ivan Rehor
- University of Chemistry and Technology Prague, Faculty of Chemical Engineering, Technicka 5, 166 28 Prague 6, Czech Republic. and Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences, Flemingovo nam. 2, 160 00, Prague, Czech Republic
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10
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Simič R, Spencer ND. Controlling the Friction of Gels by Regulating Interfacial Oxygen During Polymerization. TRIBOLOGY LETTERS 2021; 69:86. [PMID: 34776715 PMCID: PMC8550623 DOI: 10.1007/s11249-021-01459-1] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 03/31/2021] [Accepted: 05/17/2021] [Indexed: 05/15/2023]
Abstract
Hydrogel surfaces are of great interest in applications ranging from cell scaffolds and transdermal drug-delivery patches to catheter coatings and contact lenses. In this work, we propose a method to control the surface structure of hydrogels, thereby tailoring their frictional properties. The method is based on oxygen inhibition of the free-radical polymerization reaction during synthesis and enables (i) control of friction over more than an order in magnitude and (ii) spatial control of friction as either a continuous gradient or a distinct pattern. The presented method has successfully been applied to acrylamide-, diacrylate- and methacrylate-based gels, illustrating the universality of the presented method, and its potential use in the above-mentioned applications.
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Affiliation(s)
- Rok Simič
- Laboratory for Surface Science and Technology, Department of Materials, ETH Zürich, Zurich, Switzerland
| | - Nicholas D. Spencer
- Laboratory for Surface Science and Technology, Department of Materials, ETH Zürich, Zurich, Switzerland
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11
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Porte E, Cann P, Masen M. A lubrication replenishment theory for hydrogels. SOFT MATTER 2020; 16:10290-10300. [PMID: 33047773 DOI: 10.1039/d0sm01236j] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/25/2023]
Abstract
Hydrogels are suggested as less invasive alternatives to total joint replacements, but their inferior tribological performance compared to articular cartilage remains a barrier to implementation. Existing lubrication theories do not fully characterise the friction response of all hydrogels, and a better insight into the lubrication mechanisms must be established to enable optimised hydrogel performance. We therefore studied the lubricating conditions in a hydrogel contact using fluorescent imaging under simulated physiological sliding conditions. A reciprocating configuration was used to examine the effects of contact dimension and stroke length on the lubricant replenishment in the contact. The results show that the lubrication behaviour is strongly dependent on the contact configurations; When the system operates in a 'migrating' configuration, with the stroke length larger than the contact width, the contact is uniformly lubricated and shows low friction; When the contact is in an 'overlapping' configuration with a stroke length smaller than the contact width, the contact is not fully replenished, resulting in high friction. The mechanism of non-replenishment at small relative stroke length was also observed in a cartilage contact, indicating that the theory could be generalised to soft porous materials. The lubrication replenishment theory is important for the development of joint replacement materials, as most physiological joints operate under conditions of overlapping contact, meaning steady-state lubrication does not necessarily occur.
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Affiliation(s)
- Elze Porte
- Tribology Group, Department of Mechanical Engineering, Imperial College London, SW7 2AZ, UK
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12
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Bonyadi SZ, Demott CJ, Grunlan MA, Dunn AC. Cartilage-like tribological performance of charged double network hydrogels. J Mech Behav Biomed Mater 2020; 114:104202. [PMID: 33243694 DOI: 10.1016/j.jmbbm.2020.104202] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/30/2020] [Revised: 10/16/2020] [Accepted: 11/06/2020] [Indexed: 10/23/2022]
Abstract
A synthetic hydrogel material may offer utility as a cartilage replacement if it is able to maintain low friction in different sliding environments and achieve bulk mechanical properties to withstand the severe environment of the joint. In this work, we compared the tribological behavior of four double network (DN) hydrogels to that of fresh porcine cartilage in both water and fetal bovine serum (FBS). The DN hydrogels were comprised of a negatively charged 1st network and a 2nd network wherein comonomers of varying charge (i.e. neutral, positive, negative, and zwitterionic) were introduced at 10 wt% to an otherwise neutral network. A steel ball probe was used to perform microindentation tests to determine the surface elastic modulus of the samples and estimate their contact areas during sliding. Friction tests using a stationary probe with a stage that reciprocated at a range of speeds were performed to develop lubrication curves in both water and FBS. We found that the DN hydrogels with a neutral or zwitterionic 2nd network had the lowest friction and shear stresses, notably below that of cartilage. The differences in charge and structure of the samples were more evident in water than in FBS, as the lubrication responses for all the hydrogels spanned a wider range of values. In FBS, the lubrication responses were pushed towards elasto-hydrodynamics with nearly all friction coefficient values falling below 0.3. This indicates that the FBS interacts with the hydrogels and cartilage samples in a similar manner as that of cartilage by maintaining a robust layer of solution at the interface during sliding. These DN hydrogels prove to fulfill, and in some cases surpass, the lubrication demands for cartilage replacement in load bearing joints.
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Affiliation(s)
- Shabnam Z Bonyadi
- Department of Mechanical Science & Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, USA
| | - Connor J Demott
- Department of Biomedical Engineering, Texas A&M University, College Station, TX, USA
| | - Melissa A Grunlan
- Department of Biomedical Engineering, Texas A&M University, College Station, TX, USA; Department of Materials Science & Engineering, Texas A&M University, College Station, TX, USA; Department of Chemistry, Texas A&M University, College Station, TX, USA
| | - Alison C Dunn
- Department of Mechanical Science & Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, USA.
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13
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Abstract
Since their inception, hydrogels have gained popularity among multiple fields, most significantly in biomedical research and industry. Due to their resemblance to biological tribosystems, a significant amount of research has been conducted on hydrogels to elucidate biolubrication mechanisms and their possible applications as replacement materials. This review is focused on lubrication mechanisms and covers friction models that have attempted to quantify the complex frictional characteristics of hydrogels. From models developed on the basis of polymer physics to the concept of hydration lubrication, assumptions and conditions for their applicability are discussed. Based on previous models and our own experimental findings, we propose the viscous-adhesive model for hydrogel friction. This model accounts for the effects of confinement of the polymer network provided by a solid surface and poroelastic relaxation as well as the (non) Newtonian shear of a complex fluid on the frictional force and quantifies the frictional response of hydrogels-solid interfaces. Finally, the review delineates potential areas of future research based on the current knowledge.
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14
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Xiang L, Zhang J, Wang W, Gong L, Zhang L, Yan B, Zeng H. Nanomechanics of π-cation-π interaction with implications for bio-inspired wet adhesion. Acta Biomater 2020; 117:294-301. [PMID: 33007483 DOI: 10.1016/j.actbio.2020.09.043] [Citation(s) in RCA: 15] [Impact Index Per Article: 3.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/29/2020] [Revised: 09/18/2020] [Accepted: 09/22/2020] [Indexed: 12/14/2022]
Abstract
Cation-π interactions play a vital role in modulating various biological processes, e.g., potassium-selective channel, protein folding and adhesion of marine organism. Previous studies mainly focus on binary cation-π interaction, whereas due to the complexity of biological systems and surrounding environments, a single cation is often in close proximity with more than one π-conjugated unit, which could exhibit essentially different binding behavior. Herein, the first experimental evidence of ternary π-cation-π interaction is reported through direct nanomechanical force measurement in a model π-conjugated poly(catechol) (PC) system coexisting with K+. Ternary π-cation-π interactions can bridge π-conjugated moieties, resulting in robust adhesion and promoting PC assembly and deposition. Particularly, these ternary complexes are discovered to transit to binary binding pairs by increasing K+ concentration, undermining adhesion and assembly due to lack of bridging. The π-cation-π binding strength follows the trend of NMe4+ > K+ > Na+ > Li+. Employing the π-cation-π interaction, a deposition strategy to fabricate π-conjugated moiety based adhesive coatings on different substrates is realized. Our findings provide useful insights in engineering wet adhesives and coatings with reversible adhesion properties, and more broadly, with implications on rationalizing biological assembly.
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Affiliation(s)
- Li Xiang
- Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 1H9, Canada
| | - Jiawen Zhang
- Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 1H9, Canada
| | - Wenda Wang
- Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 1H9, Canada
| | - Lu Gong
- Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 1H9, Canada
| | - Ling Zhang
- Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 1H9, Canada
| | - Bin Yan
- Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 1H9, Canada; College of Light Industry, Textile & Food Engineering, Sichuan University, Chengdu 610065, China
| | - Hongbo Zeng
- Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta, T6G 1H9, Canada.
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15
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Yi J, Nguyen KCT, Wang W, Yang W, Pan M, Lou E, Major PW, Le LH, Zeng H. Polyacrylamide/Alginate double-network tough hydrogels for intraoral ultrasound imaging. J Colloid Interface Sci 2020; 578:598-607. [DOI: 10.1016/j.jcis.2020.06.015] [Citation(s) in RCA: 20] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/27/2020] [Revised: 05/24/2020] [Accepted: 06/03/2020] [Indexed: 12/20/2022]
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16
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Bhattacharyya A, O'Bryan C, Ni Y, Morley CD, Taylor CR, Angelini TE. Hydrogel compression and polymer osmotic pressure. ACTA ACUST UNITED AC 2020. [DOI: 10.1016/j.biotri.2020.100125] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/24/2022]
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17
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Cuccia NL, Pothineni S, Wu B, Méndez Harper J, Burton JC. Pore-size dependence and slow relaxation of hydrogel friction on smooth surfaces. Proc Natl Acad Sci U S A 2020; 117:11247-11256. [PMID: 32398363 PMCID: PMC7260953 DOI: 10.1073/pnas.1922364117] [Citation(s) in RCA: 30] [Impact Index Per Article: 7.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Hydrogels consist of a cross-linked polymer matrix imbibed with a solvent such as water at volume fractions that can exceed 90%. They are important in many scientific and engineering applications due to their tunable physiochemical properties, biocompatibility, and ultralow friction. Their multiphase structure leads to a complex interfacial rheology, yet a detailed, microscopic understanding of hydrogel friction is still emerging. Using a custom-built tribometer, here we identify three distinct regimes of frictional behavior for polyacrylic acid (PAA), polyacrylamide (PAAm), and agarose hydrogel spheres on smooth surfaces. We find that at low velocities, friction is controlled by hydrodynamic flow through the porous hydrogel network and is inversely proportional to the characteristic pore size. At high velocities, a mesoscopic, lubricating liquid film forms between the gel and surface that obeys elastohydrodynamic theory. Between these regimes, the frictional force decreases by an order of magnitude and displays slow relaxation over several minutes. Our results can be interpreted as an interfacial shear thinning of the polymers with an increasing relaxation time due to the confinement of entanglements. This transition can be tuned by varying the solvent salt concentration, solvent viscosity, and sliding geometry at the interface.
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Affiliation(s)
| | | | - Brady Wu
- Department of Physics, Emory University, Atlanta, GA 30322
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18
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Chau AL, Rosas J, Degen GD, Månsson LK, Chen J, Valois E, Pitenis AA. Aqueous surface gels as low friction interfaces to mitigate implant-associated inflammation. J Mater Chem B 2020; 8:6782-6791. [PMID: 32364211 DOI: 10.1039/d0tb00582g] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/23/2022]
Abstract
Aqueous surface gels are fragile yet resilient biopolymer-based networks capable of sustaining extremely low friction coefficients despite tribologically-challenging environments. These superficial networks are ubiquitous in natural sliding interfaces and protect mechanosensitive cells from excessive contact pressures and frictional shear stresses from cell-fluid, cell-cell, or cell-solid interactions. Understanding these complex lubrication mechanisms may aid in the development of materials-based strategies for increasing biocompatibility in medical devices and implants. Equally as important is characterizing the interplay between soft and passive yet mobile implant materials and cellular reactions in response to direct contact and frictional shear stresses. Physically interrogating living biological systems without rupturing them in the process is nontrivial. To this end, custom biotribometers have been designed to precisely modulate contact pressures against living human telomerase-immortalized corneal epithelial (hTCEpi) cell layers using soft polyacrylamide membrane probes. Reverse-transcription quantitative polymerase chain-reaction (RT-qPCR) indicated that increased duration and, to a much greater extent, the magnitude of frictional shear stress lead to increased production of pro-inflammatory (IL-1β, IL-6, MMP9) and pro-apoptotic (DDIT3, FAS) genes, which in clinical studies are linked to pathological pain. The hierarchical structure often found in biological systems has also been investigated through the fabrication of high-water content (polyacrylamide) hydrogels through free-radical polymerization inhibition. Nanoindentation experiments and friction coefficient measurements indicate that these "gradient surface gels" reduce contact pressures and frictional shear stresses at the surface of the material while still maintaining stiffness within the bulk. Reducing frictional shear stresses through informed materials and surface design may concomitantly increase lubricity and quiet the immune response, and thus provide bio-inspired routes to improve patient outcomes and quality of life.
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Affiliation(s)
- Allison L Chau
- Materials Department University of California, Santa Barbara, CA 93106, USA.
| | - Jonah Rosas
- Biomolecular Science and Engineering Department University of California, Santa Barbara, CA 93106, USA
| | - George D Degen
- Department of Chemical Engineering, University of California, Santa Barbara Santa Barbara, CA 93106, USA
| | - Lisa K Månsson
- Department of Physics Chalmers, University of Technology, 412 58 Gothenburg, Sweden
| | - Jonathan Chen
- Department of Chemical Engineering, University of California, Santa Barbara Santa Barbara, CA 93106, USA
| | - Eric Valois
- Department of Molecular, Cellular, and Developmental Biology, University of California, Santa Barbara, CA 93106, USA
| | - Angela A Pitenis
- Materials Department University of California, Santa Barbara, CA 93106, USA.
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19
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Rudge RED, van de Sande JPM, Dijksman JA, Scholten E. Uncovering friction dynamics using hydrogel particles as soft ball bearings. SOFT MATTER 2020; 16:3821-3831. [PMID: 32248205 DOI: 10.1039/d0sm00080a] [Citation(s) in RCA: 12] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/11/2023]
Abstract
Rolling ball bearings are widely known and applied to decrease friction between two surfaces. More recently, hydrogel-hydrogel tribopairs have also revealed good but rather complex lubrication properties. Here, we use hydrogels as ball bearings to elucidate that soft spherical particles have nontrivial rate-dependent lubrication behavior. Unlike Newtonian lubrication or dry solid friction, hydrogel particles in suspension transition through four frictional regimes as a function of sliding velocity. We relate the different regimes to the deformation of the particles at different gap sizes, which changes the effective contact area between the sliding surfaces. By systematically varying the particle characteristics and the surface properties of the sliding surfaces, we assign potential mechanisms for each of the different lubricating regimes as a function of velocity: (I) relatively high friction due to particle flattening and direct contact between interacting bodies (II) decrease of friction owing to the presence of rolling particles (III) large inflow of particles in a confined space leading to compressed particles and (IV) the formation of a thick lubricating layer. Using these suspensions with soft, deformable particles as a ball bearing system, we provide new insights into soft material friction with applications in emulsions, powders, pastes or other granular materials.
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Affiliation(s)
- Raisa E D Rudge
- Physics and Physical Chemistry of Foods, Wageningen University, The Netherlands. and Physical Chemistry and Soft Matter, Wageningen University, The Netherlands
| | | | - Joshua A Dijksman
- Physical Chemistry and Soft Matter, Wageningen University, The Netherlands
| | - Elke Scholten
- Physics and Physical Chemistry of Foods, Wageningen University, The Netherlands.
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20
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Irwin NJ, Bryant MG, McCoy CP, Trotter JL, Turner J. Multifunctional, Low Friction, Antimicrobial Approach for Biomaterial Surface Enhancement. ACS APPLIED BIO MATERIALS 2020; 3:1385-1393. [PMID: 35021631 DOI: 10.1021/acsabm.9b01042] [Citation(s) in RCA: 9] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Poly(vinyl chloride) (PVC) biomaterials perform a host of life-saving and life-enhancing roles when employed as medical devices within the body. High frictional forces between the device surface and interfacing tissue can, however, lead to a host of complications including tissue damage, inflammation, pain, and infection. We herein describe a versatile surface modification method using multifunctional hydrogel formulations to increase lubricity and prevent common device-related complications. In a clinically relevant model of the urinary tract, simulating the mechanical and biological environments encountered in vivo, coated candidate catheter surfaces demonstrated significantly lower frictional resistance than uncoated PVC, with reductions in coefficient of friction values of more than 300-fold due to hydration of the surface-localized polymer network. Furthermore, this significant lubrication capacity was retained following hydration periods of up to 28 days in artificial urine at pH 6 and pH 9, representing the pH of physiologically normal and infected urine, respectively, and during 200 repeated cycles of applied frictional force. Importantly, the modified surfaces also displayed excellent antibacterial activity, which could be facilely tuned to achieve reductions of 99.8% in adherence of common hospital-acquired pathogens, Staphylococcus aureus and Proteus mirabilis, relative to their uncoated counterparts through incorporation of chlorhexidine in the coating matrix as a model antiseptic. The remarkable, and pH-independent, tribological performance of these lubricious, antibacterial, and highly durable surfaces offers exciting promise for use of this PVC functionalization approach in facilitating smooth and atraumatic insertion and removal of a wide range of medical implants, ultimately maintaining user health and dignity.
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Affiliation(s)
- Nicola J Irwin
- School of Pharmacy, Queen's University Belfast, 97 Lisburn Road, Belfast BT9 7BL, Northern Ireland, U.K
| | - Michael G Bryant
- School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom
| | - Colin P McCoy
- School of Pharmacy, Queen's University Belfast, 97 Lisburn Road, Belfast BT9 7BL, Northern Ireland, U.K
| | - Johann L Trotter
- School of Pharmacy, Queen's University Belfast, 97 Lisburn Road, Belfast BT9 7BL, Northern Ireland, U.K
| | - Jonathan Turner
- School of Pharmacy, Queen's University Belfast, 97 Lisburn Road, Belfast BT9 7BL, Northern Ireland, U.K
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21
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Li H, Choi YS, Rutland MW, Atkin R. Nanotribology of hydrogels with similar stiffness but different polymer and crosslinker concentrations. J Colloid Interface Sci 2020; 563:347-353. [PMID: 31887698 DOI: 10.1016/j.jcis.2019.12.045] [Citation(s) in RCA: 12] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/21/2019] [Revised: 12/02/2019] [Accepted: 12/12/2019] [Indexed: 11/26/2022]
Abstract
HYPOTHESIS The stiffness has been found to regulate hydrogel performances and applications. However, the key interfacial properties of hydrogels, like friction and adhesion are not controlled by the stiffness, but are altered by the structure and composition of hydrogels, like polymer volume fraction and crosslinking degree. EXPERIMENTS Colloidal probe atomic force microscopy has been use to investigate the relationship between tribological properties (friction and adhesion) and composition of hydrogels with similar stiffness, but different polymer volume fractions and crosslinking degrees. FINDINGS The interfacial normal and lateral (friction) forces of hydrogels are not directly correlated to the stiffness, but altered by the hydrogel structure and composition. For normal force measurements, the adhesion increases with polymer volume fraction but decreases with crosslinking degree. For lateral force measurements, friction increases with polymer volume fraction, but decreases with crosslinking degree. In the low normal force regime, friction is mainly adhesion-controlled and increases significantly with the adhesion and polymer volume fraction. In the high normal force regime, friction is predominantly load-controlled and shows slow increase with normal force.
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Affiliation(s)
- Hua Li
- School of Molecular Sciences, The University of Western Australia, 35 Stirling Highway, Perth, WA 6009, Australia; Centre for Microscopy, Characterisation and Analysis, The University of Western Australia, Perth, WA 6009, Australia.
| | - Yu Suk Choi
- School of Human Sciences, The University of Western Australia, 35 Stirling Highway, Perth, WA 6009, Australia
| | - Mark W Rutland
- School of Chemical Science and Engineering, KTH Royal Institute of Technology, SE100 44, Sweden; Surfaces, Processes and Formulation, RISE Research Institutes of Sweden, SE114 86 Stockholm, Sweden
| | - Rob Atkin
- School of Molecular Sciences, The University of Western Australia, 35 Stirling Highway, Perth, WA 6009, Australia
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22
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Meier YA, Zhang K, Spencer ND, Simic R. Linking Friction and Surface Properties of Hydrogels Molded Against Materials of Different Surface Energies. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2019; 35:15805-15812. [PMID: 31369280 PMCID: PMC6899455 DOI: 10.1021/acs.langmuir.9b01636] [Citation(s) in RCA: 30] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/02/2023]
Abstract
Biological tissues subjected to rubbing, such as the cornea and eyelid or articular cartilage, are covered in brushy, hydrated mucous structures in order to reduce the shear stress on the tissue. To mimic such biological tissues, we have prepared polyacrylamide (PAAm) hydrogels with various concentrations of un-cross-linked chains on their surfaces by synthesizing them in molds of different surface energies. The selected molding materials included hydrophilic glass, polyoxymethylene (POM), polystyrene (PS), polyethylene (PE), polypropylene (PP), and polytetrafluoroethylene (PTFE). After synthesis, demolding, and equilibration in water, the elastic modulus at the hydrogel surface decreased with increasing water contact angle of the mold. The softer, brushier surfaces did not completely collapse under compressive pressures up to 10 kPa, remaining better hydrated compared to their denser, cross-linked analogs. The hydrogels with brushier surfaces displayed an order of magnitude lower coefficient of friction than the cross-linked ones, which is attributed to the ability of their near-surface regions to retain larger amounts of liquid at the interface. The characteristic speed-dependent friction of the denser, cross-linked hydrogel surface is compared to the speed-independent friction of the brushy hydrogels and discussed from the perspectives of (elasto)hydrodynamic lubrication, permeability, and shear-induced hydrodynamic penetration depth.
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23
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McGhee EO, Hart SM, Urueña JM, Sawyer WG. Hydration Control of Gel-Adhesion and Muco-Adhesion. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2019; 35:15769-15775. [PMID: 31659909 DOI: 10.1021/acs.langmuir.9b02816] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Protective mucin gel layers established by epithelial cell surfaces in biology have water contents above 90% and provide a low-shear stress nonadhesive interfacial boundary on epithelial surfaces throughout the body. Adhesion between gels and mucin layers, muco-adhesion, is an important aspect of drug delivery, biocompatibility, and the prevention of damage during insertion, use, and removal of medical devices in contact with moist epithelial surfaces. This manuscript develops a simple mathematical model to suggest that gel-adhesion and muco-adhesion are controlled by dehydration. For a fully swollen gel, the osmotic pressure is balanced by the elastic stress in the polymer gel, and differences in the elastic modulus are used to calculate dehydration stresses. A model based on Winkler contact mechanics gives a closed form expression for the force of adhesion that is dependent on the contact radius and gel thickness, inversely proportional to the mucin layer stiffness, and proportional to the square of the differences in elastic modulus. Submerged contact experiments conducted on Gemini gel interfaces of polyacrylamide aqueous gels showed increasing adhesion with increasing dehydration of the probe. Additionally, experiments conducted against mucinated epithelial cell monolayers found mucin transfer onto the most dehydrated gels and no transfer on swollen gels. The model and experiments reveal that high water content fully swollen gels are not intrinsically muco-adhesive, which is consistent with previous tribological experience showing increased lubricity with increasing water content and mesh size.
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Affiliation(s)
- Eric O McGhee
- Department of Mechanical and Aerospace Engineering , University of Florida Gainesville , FL 32611 , United States
| | - Samuel M Hart
- Department of Mechanical and Aerospace Engineering , University of Florida Gainesville , FL 32611 , United States
| | - Juan Manuel Urueña
- Department of Mechanical and Aerospace Engineering , University of Florida Gainesville , FL 32611 , United States
| | - W Gregory Sawyer
- Department of Mechanical and Aerospace Engineering , University of Florida Gainesville , FL 32611 , United States
- J. Crayton Pruitt Family Department of Biomedical Engineering , University of Florida Gainesville , FL 32611 , United States
- Department of Materials Science and Engineering , University of Florida Gainesville , FL 32611 , United States
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24
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Bonyadi SZ, Atten M, Dunn AC. Self-regenerating compliance and lubrication of polyacrylamide hydrogels. SOFT MATTER 2019; 15:8728-8740. [PMID: 31553022 DOI: 10.1039/c9sm01607d] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
Pristine hydrogel surfaces typically have low friction, which is controlled by composition, slip speeds, and immediate slip history. The stiffness of such samples is typically measured with bulk techniques, and is assumed to be homogeneous at the surface. While the surface properties of homogeneous hydrogel samples are generally controlled by composition, the surface also interfaces with the open bath, which distinguishes it from the bulk. In this work, we disrupt as-molded polyacrylamide surfaces with abrasive wear and connect the effects on the surface stiffness and lubrication to the wear events. At both the nanoscale and the microscale, quasistatic indentations reveal a stiffer surface by up to two times following wear events, even considering roughness. Longitudinal experiments with a series of wear episodes interposed with periods of re-equilibration show that increased stiffness is reversible: more compliant surfaces regenerate within 24 hours. The timescale suggests an osmotic swelling mechanism, and we postulate that abrasive wear removes a swollen surface layer, revealing the stiffer bulk. The newly-revealed bulk becomes the surface, which re-swells over time. We quantify the effects on the self-lubricating ability of these surfaces following abrasive wear using micro-tribometry. The lubrication curve shows that robust low friction is maintained, and that the friction becomes less dependent upon the sliding speed. The unique ability of these materials to regenerate swollen surfaces and maintain robust low friction following abrasive wear is promising for designing their slip behavior into aqueous soft robotics components or biomedicine applications.
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Affiliation(s)
- Shabnam Z Bonyadi
- Department of Mechanical Science & Engineering, University of Illinois at Urbana-Champaign, MechSE @ UIUC, 1206 W Green St, MC 244, Urbana, IL 61801, USA.
| | - Michael Atten
- Department of Mechanical Science & Engineering, University of Illinois at Urbana-Champaign, MechSE @ UIUC, 1206 W Green St, MC 244, Urbana, IL 61801, USA.
| | - Alison C Dunn
- Department of Mechanical Science & Engineering, University of Illinois at Urbana-Champaign, MechSE @ UIUC, 1206 W Green St, MC 244, Urbana, IL 61801, USA.
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25
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Rudge RED, Scholten E, Dijksman JA. Advances and challenges in soft tribology with applications to foods. Curr Opin Food Sci 2019. [DOI: 10.1016/j.cofs.2019.06.011] [Citation(s) in RCA: 27] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/16/2022]
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26
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Albers PT, Govers SP, Laven J, van der Ven LG, van Benthem RA, de With G, Esteves ACC. Design of dual hydrophobic–hydrophilic polymer networks for highly lubricious polyether-urethane coatings. Eur Polym J 2019. [DOI: 10.1016/j.eurpolymj.2018.12.004] [Citation(s) in RCA: 10] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
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27
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Fluid load support does not explain tribological performance of PVA hydrogels. J Mech Behav Biomed Mater 2019; 90:284-294. [DOI: 10.1016/j.jmbbm.2018.09.048] [Citation(s) in RCA: 13] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/03/2018] [Revised: 08/22/2018] [Accepted: 09/30/2018] [Indexed: 11/29/2022]
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28
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Langowski JKA, Rummenie A, Pieters RPM, Kovalev A, Gorb SN, van Leeuwen JL. Estimating the maximum attachment performance of tree frogs on rough substrates. BIOINSPIRATION & BIOMIMETICS 2019; 14:025001. [PMID: 30706849 DOI: 10.1088/1748-3190/aafc37] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/05/2023]
Abstract
Tree frogs can attach to smooth and rough substrates using their adhesive toe pads. We present the results of an experimental investigation of tree frog attachment to rough substrates, and of the role of mechanical interlocking between superficial toe pad structures and substrate asperities in the tree frog species Litoria caerulea and Hyla cinerea. Using a rotation platform setup, we quantified the adhesive and frictional attachment performance of whole frogs clinging to smooth, micro-, and macrorough substrates. The transparent substrates enabled quantification of the instantaneous contact area during detachment by using frustrated total internal reflection. A linear mixed-effects model shows that the adhesive performance of the pads does not differ significantly with roughness (for nominal roughness levels of 0-15 µm) in both species. This indicates that mechanical interlocking does not contribute to the attachment of whole animals. Our results show that the adhesion performance of tree frogs is higher than reported previously, emphasising the biomimetic potential of tree frog attachment. Overall, our findings contribute to a better understanding of the complex interplay of attachment mechanisms in the toe pads of tree frogs, which may promote future designs of tree-frog-inspired adhesives.
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Affiliation(s)
- Julian K A Langowski
- Experimental Zoology Group, Wageningen University & Research, Wageningen, The Netherlands
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29
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30
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Shinomiya K, Mayama H, Nonomura Y. Anomalous Friction between Agar Gels under Accelerated Motion. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2018; 34:12723-12729. [PMID: 30272977 DOI: 10.1021/acs.langmuir.8b02251] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Understanding the friction phenomena on a gel surface under accelerated conditions is important for the designing of functional materials. However, there are few reports on friction under such conditions. In the present study, the effects of velocity, normal force, and gel hardness on the friction force were evaluated between two agar gels under sinusoidal motion. We found a friction phenomenon with an extremely low friction coefficient on the gel surfaces: the friction coefficient became less than 0.02 when sliding velocity increased. In addition, the profile of the friction coefficient was different between outward and homeward processes in the reciprocating sliding motion. In the outward direction, the low friction coefficient was maintained even if the sliding velocity decreased. On the other hand, the friction coefficient increased with sliding velocity in the homeward direction. This characteristic friction profile is caused by a long relaxation time on the gel surfaces. When the gel substrate is rubbed for a shorter time than the relaxation time, the morphology of the gel surface becomes unstable. Under such conditions, the formation and extinction of a thick liquid film can induce a super lubrication state and the asymmetric friction phenomena. These findings are useful not only for developing functional materials but also for understanding nonequilibrium phenomena in soft biological systems.
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Affiliation(s)
- Koki Shinomiya
- Department of Biochemical Engineering, Graduate School of Science and Engineering , Yamagata University , 4-3-16 Jonan , Yonezawa 992-8510 , Japan
| | - Hiroyuki Mayama
- Department of Chemistry , Asahikawa Medical University , 2-1-1-1 Midorigaoka-Higashi , Asahikawa 078-8510 , Japan
| | - Yoshimune Nonomura
- Department of Biochemical Engineering, Graduate School of Science and Engineering , Yamagata University , 4-3-16 Jonan , Yonezawa 992-8510 , Japan
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31
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Langowski JKA, Dodou D, Kamperman M, van Leeuwen JL. Tree frog attachment: mechanisms, challenges, and perspectives. Front Zool 2018; 15:32. [PMID: 30154908 PMCID: PMC6107968 DOI: 10.1186/s12983-018-0273-x] [Citation(s) in RCA: 41] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/08/2017] [Accepted: 06/29/2018] [Indexed: 11/16/2022] Open
Abstract
Tree frogs have the remarkable ability to attach to smooth, rough, dry, and wet surfaces using their versatile toe pads. Tree frog attachment involves the secretion of mucus into the pad-substrate gap, requiring adaptations towards mucus drainage and pad lubrication. Here, we present an overview of tree frog attachment, with focus on (i) the morphology and material of the toe pad; (ii) the functional demands on the toe pad arising from ecology, lifestyle, and phylogenetics; (iii) experimental data of attachment performance such as adhesion and friction forces; and (iv) potential perspectives on future developments in the field. By revisiting reported data and observations, we discuss the involved mechanisms of attachment and propose new hypotheses for further research. Among others, we address the following questions: Do capillary and hydrodynamic forces explain the strong friction of the toe pads directly, or indirectly by promoting dry attachment mechanisms? If friction primarily relies on van der Waals (vdW) forces instead, how much do these forces contribute to adhesion in the wet environment tree frogs live in and what role does the mucus play? We show that both pad morphology and measured attachment performance suggest the coaction of several attachment mechanisms (e.g. capillary and hydrodynamic adhesion, mechanical interlocking, and vdW forces) with situation-dependent relative importance. Current analytical models of capillary and hydrodynamic adhesion, caused by the secreted mucus and by environmental liquids, do not capture the contributions of these mechanisms in a comprehensive and accurate way. We argue that the soft pad material and a hierarchical surface pattern on the ventral pad surface enhance the effective contact area and facilitate gap-closure by macro- to nanoscopic drainage of interstitial liquids, which may give rise to a significant contribution of vdW interactions to tree frog attachment. Increasing the comprehension of the complex mechanism of tree frog attachment contributes to a better understanding of other biological attachment systems (e.g. in geckos and insects) and is expected to stimulate the development of a wide array of bioinspired adhesive applications.
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Affiliation(s)
- Julian K. A. Langowski
- Experimental Zoology Group, Department of Animal Sciences, Wageningen University & Research, De Elst 1, Wageningen, 6708 WD The Netherlands
| | - Dimitra Dodou
- Department of BioMechanical Engineering, Faculty of Mechanical, Maritime and Materials Engineering, Delft University of Technology, Mekelweg 2, Delft, 2628 CD The Netherlands
| | - Marleen Kamperman
- Physical Chemistry and Soft Matter, Department of Agrotechnology and Food Sciences, Wageningen University & Research, Stippeneng 4, Wageningen, 6708 WE The Netherlands
| | - Johan L. van Leeuwen
- Experimental Zoology Group, Department of Animal Sciences, Wageningen University & Research, De Elst 1, Wageningen, 6708 WD The Netherlands
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32
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Urueña JM, McGhee EO, Angelini TE, Dowson D, Sawyer WG, Pitenis AA. Normal Load Scaling of Friction in Gemini Hydrogels. ACTA ACUST UNITED AC 2018. [DOI: 10.1016/j.biotri.2018.01.002] [Citation(s) in RCA: 19] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/18/2022]
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33
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Bhattacharjee T, Kabb CP, O'Bryan CS, Urueña JM, Sumerlin BS, Sawyer WG, Angelini TE. Polyelectrolyte scaling laws for microgel yielding near jamming. SOFT MATTER 2018; 14:1559-1570. [PMID: 29450413 DOI: 10.1039/c7sm01518f] [Citation(s) in RCA: 26] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/08/2023]
Abstract
Micro-scale hydrogel particles, known as microgels, are used in industry to control the rheology of numerous different products, and are also used in experimental research to study the origins of jamming and glassy behavior in soft-sphere model systems. At the macro-scale, the rheological behaviour of densely packed microgels has been thoroughly characterized; at the particle-scale, careful investigations of jamming, yielding, and glassy-dynamics have been performed through experiment, theory, and simulation. However, at low packing fractions near jamming, the connection between microgel yielding phenomena and the physics of their constituent polymer chains has not been made. Here we investigate whether basic polymer physics scaling laws predict macroscopic yielding behaviours in packed microgels. We measure the yield stress and cross-over shear-rate in several different anionic microgel systems prepared at packing fractions just above the jamming transition, and show that our data can be predicted from classic polyelectrolyte physics scaling laws. We find that diffusive relaxations of microgel deformation during particle re-arrangements can predict the shear-rate at which microgels yield, and the elastic stress associated with these particle deformations predict the yield stress.
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Shoaib T, Heintz J, Lopez-Berganza JA, Muro-Barrios R, Egner SA, Espinosa-Marzal RM. Stick-Slip Friction Reveals Hydrogel Lubrication Mechanisms. LANGMUIR : THE ACS JOURNAL OF SURFACES AND COLLOIDS 2018; 34:756-765. [PMID: 28961012 DOI: 10.1021/acs.langmuir.7b02834] [Citation(s) in RCA: 32] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/18/2023]
Abstract
The lubrication behavior of the hydrated biopolymers that constitute tissues in organisms differs from that outlined by the classical Stribeck curve, and studying hydrogel lubrication is a key pathway to understand the complexity of biolubrication. Here, we have investigated the frictional characteristics of polyacrylamide (PAAm) hydrogels with various acrylamide concentrations, exhibiting Young's moduli (E) that range from 1 to 40 kPa, as a function of applied normal load and sliding velocities by colloid probe lateral force microscopy. The speed-dependence of the friction force shows an initial decrease in friction with increasing velocity, while, above a transition velocity V*, friction increases with speed. This study reveals two different boundary lubrication mechanisms characterized by distinct scaling laws. An unprecedented and comprehensive study of the lateral force loops reveals intermittent friction or stick-slip above and below V*, with characteristics that depend on the hydrogel network, applied load, and sliding velocity. Our work thus provides insight into the closely tied parameters governing hydrogel lubrication mechanisms, and stick-slip friction.
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Affiliation(s)
- Tooba Shoaib
- Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign , 205 North Matthews Avenue, Urbana, Illinois 61801, United States
- Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign , 1304 West Green Street, Urbana, Illinois 61801, United States
| | - Joerg Heintz
- Health Care Engineering Systems Center, University of Illinois at Urbana-Champaign , 1206 West Clark Street, Urbana, Illinois 61801, United States
| | - Josue A Lopez-Berganza
- Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign , 205 North Matthews Avenue, Urbana, Illinois 61801, United States
| | - Raymundo Muro-Barrios
- Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign , 1304 West Green Street, Urbana, Illinois 61801, United States
| | - Simon A Egner
- Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign , 1304 West Green Street, Urbana, Illinois 61801, United States
| | - Rosa M Espinosa-Marzal
- Department of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign , 205 North Matthews Avenue, Urbana, Illinois 61801, United States
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Pitenis AA, Urueña JM, Hormel TT, Bhattacharjee T, Niemi SR, Marshall SL, Hart SM, Schulze KD, Angelini TE, Sawyer WG. Corneal cell friction: Survival, lubricity, tear films, and mucin production over extended duration in vitro studies. ACTA ACUST UNITED AC 2017. [DOI: 10.1016/j.biotri.2017.04.003] [Citation(s) in RCA: 27] [Impact Index Per Article: 3.9] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
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Iuster N, Tairy O, Driver MJ, Armes SP, Klein J. Cross-Linking Highly Lubricious Phosphocholinated Polymer Brushes: Effect on Surface Interactions and Frictional Behavior. Macromolecules 2017. [DOI: 10.1021/acs.macromol.7b01423] [Citation(s) in RCA: 31] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Affiliation(s)
- Noa Iuster
- Department
of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel
| | - Odeya Tairy
- Department
of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel
| | - Michael J. Driver
- Vertellus Biomaterials,
Vertellus Specialties UK Ltd., Basingstoke, Hampshire RG25 2PH, U.K
| | - Steven P. Armes
- Department
of Chemistry, University of Sheffield, Sheffield S3 7HF, U.K
| | - Jacob Klein
- Department
of Materials and Interfaces, Weizmann Institute of Science, Rehovot 76100, Israel
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Kim J, Dunn AC. Soft hydrated sliding interfaces as complex fluids. SOFT MATTER 2016; 12:6536-6546. [PMID: 27425448 DOI: 10.1039/c6sm00623j] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/06/2023]
Abstract
Hydrogel surfaces are biomimics for sensing and mobility systems in the body such as the eyes and large joints due to their important characteristics of flexibility, permeability, and integrated aqueous component. Recent studies have shown polymer concentration gradients resulting in a less dense region in the top micrometers of the surface. Under shear, this gradient is hypothesized to drive lubrication behavior due to its rheological similarity to a semi-dilute polymer solution. In this work we map 3 distinct lubricating regimes between a polyacrylamide surface and an aluminum annulus using stepped-velocity tribo-rheometry over 5 decades of sliding speed in increasing and decreasing steps. These regimes, characterized by weakly or strongly time-dependent response and thixotropy-like hysteresis, provide the skeleton of a lubrication curve for hydrogel-against-hard material interfaces and support hypotheses of polymer mechanics-driven lubrication. Tribo-rheometry is particularly suited to uncover the lubrication mechanisms of complex interfaces such as are formed with hydrated hydrogel surfaces and biological surfaces.
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Affiliation(s)
- Jiho Kim
- Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA.
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39
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Dunn AC, Pitenis AA, Urueña JM, Schulze KD, Angelini TE, Sawyer WG. Kinetics of aqueous lubrication in the hydrophilic hydrogel Gemini interface. Proc Inst Mech Eng H 2015; 229:889-94. [DOI: 10.1177/0954411915612819] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022]
Abstract
The exquisite sliding interfaces in the human body share the common feature of hydrated dilute polymer mesh networks. These networks, especially when they constitute a sliding interface such as the pre-corneal tear film on the ocular interface, are described by the molecular weight of the polymer chains and a characteristic size of a minimum structural unit, the mesh size, ξ. In a Gemini interface where hydrophilic hydrogels are slid against each other, the aqueous lubrication behavior has been shown to be a function of sliding velocity, introducing a sliding timescale competing against the time scales of polymer fluctuation and relaxation at the surface. In this work, we examine two recent studies and postulate that when the Gemini interface slips faster than the single-chain relaxation time, chains must relax, suppressing the amplitude of the polymer chain thermal fluctuations.
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Affiliation(s)
- Alison C Dunn
- Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL, USA
| | - Angela A Pitenis
- Department of Mechanical & Aerospace Engineering, University of Florida, Gainesville, FL, USA
| | - Juan M Urueña
- Department of Mechanical & Aerospace Engineering, University of Florida, Gainesville, FL, USA
| | - Kyle D Schulze
- Department of Mechanical & Aerospace Engineering, University of Florida, Gainesville, FL, USA
| | - Thomas E Angelini
- Department of Mechanical & Aerospace Engineering, University of Florida, Gainesville, FL, USA
- J. Crayton Pruitt Family Department of Biomedical Engineering, University of Florida, Gainesville, FL, USA
- Institute for Cell Engineering and Regenerative Medicine, University of Florida, Gainesville, FL, USA
| | - W Gregory Sawyer
- Department of Mechanical & Aerospace Engineering, University of Florida, Gainesville, FL, USA
- Department of Materials Science & Engineering, University of Florida, Gainesville, FL, USA
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Yu Y, Kieviet BD, Liu F, Siretanu I, Kutnyánszky E, Vancso GJ, de Beer S. Stretching of collapsed polymers causes an enhanced dissipative response of PNIPAM brushes near their LCST. SOFT MATTER 2015; 11:8508-16. [PMID: 26371862 DOI: 10.1039/c5sm01426c] [Citation(s) in RCA: 47] [Impact Index Per Article: 5.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/05/2023]
Abstract
Poly(N-isopropyl acrylamide) (PNIPAM) is a stimulus-responsive polymer that can switch in water from an expanded state below the lower critical solution temperature (LCST) of 32 °C to a globular state above the LCST. It was recently shown that, as a consequence of this conformational transition, the interfacial and (tribo-)mechanical properties of polymeric systems composed of PNIPAM can be switched between two states. Here we show that the tribo-mechanical properties of a particular type of PNIPAM system, which is the PNIPAM brush, do not just change between two states, but instead evolve continuously and non-monotonically upon increasing/decreasing temperature. To do so, we present atomic force microscopy experiments in which we measure the adhesion hysteresis and the friction upon bringing a gold colloid in relative motion with PNIPAM brushes at temperatures around the LCST. Both the friction and the adhesion hysteresis display a pronounced maximum exactly at the LCST. The force vs. distance data captured at these temperatures show a long-ranged adhesive interaction upon moving the colloid away from the original point of contact, which indicates that during this retraction the partly collapsed polymers in the brush become strongly stretched.
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Affiliation(s)
- Yunlong Yu
- Materials Science and Technology of Polymers, MESA + Institute of Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.
| | - Bernard D Kieviet
- Materials Science and Technology of Polymers, MESA + Institute of Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.
| | - Fei Liu
- Physics of Complex Fluids, MESA + Institute of Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
| | - Igor Siretanu
- Physics of Complex Fluids, MESA + Institute of Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
| | - Edit Kutnyánszky
- Materials Science and Technology of Polymers, MESA + Institute of Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.
| | - G Julius Vancso
- Materials Science and Technology of Polymers, MESA + Institute of Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.
| | - Sissi de Beer
- Materials Science and Technology of Polymers, MESA + Institute of Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands.
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Dual responsive antibacterial Ag-poly(N-isopropylacrylamide/itaconic acid) hydrogel nanocomposites synthesized by gamma irradiation. Eur Polym J 2015. [DOI: 10.1016/j.eurpolymj.2015.06.008] [Citation(s) in RCA: 33] [Impact Index Per Article: 3.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
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Urueña JM, Pitenis AA, Nixon RM, Schulze KD, Angelini TE, Gregory Sawyer W. Mesh Size Control of Polymer Fluctuation Lubrication in Gemini Hydrogels. ACTA ACUST UNITED AC 2015. [DOI: 10.1016/j.biotri.2015.03.001] [Citation(s) in RCA: 69] [Impact Index Per Article: 7.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
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